RF devices are today used for communications in a wide number of applications from personal cellular telephones to emergency medical radios, and many more applications are envisioned for the future. In today's environment, various types of RF radios communicate with each other through a remote base station site 100 (FIG. 1) that receives and transmits communications signals to and from the various radios. For this reason, remote base station sites are strategically located around a geographic area to ensure that good communications conditions will exist between a base station site and a radio within the area.
The base stations are designed to process a multitude of conversations simultaneously, and to do so over a multitude of different communications channels. In one type of mobile radio, for example, twenty six channels are provided for communications at transmission frequencies between 896-902 MHz. Of course, the number of channels or the transmission frequencies vary depending upon the RF system currently being implemented.
As shown in FIG. 2, the base station 100 includes an antenna 200 connected to a transmission combiner 202 at the transmission end of the base station 100 and a receiver coupler 204 at the receiving end of the base station 100. The antenna 200, transmission combiner 202, and receiver coupler 204 thus combine to service a plurality of duplex RF channel transmit/receive circuits. For example, as shown in FIG. 2, the R.sub.x coupler 204 receives a signal band from antenna 200 and converts the signal band into a plurality of channels, CH1, CH2, CH3 . . . CHn, which is then directed to corresponding repeater stations 205-208.
Each repeater station 205-208 services one of the channels CH1-CHn. That is, station 205 services CH1, station 206 services CH2, etc. Each station includes transmitting circuitry "T" and receiving circuitry "R". As previously described, each receiver "R" of the stations 205-208 receives one channel signal from the R.sub.x coupler 204. Likewise, each transmitter "T" of the stations 205-208 services one channel to the T.sub.x combiner 202.
Each station transmitter and receiver is typically controlled by a dedicated control shelf CS (e.g., a microprocessor-based control circuit), as shown in FIG. 2. The control shelves CS communicate with, and are controlled by, trunking cards TC 209-212. The trunking cards TC (e.g., further microprocessor-based control circuits) communicate with one another and/or with a primary site controller 214. The primary site controller 214 provides the major intelligence and control capability for the system 100. As can be seen in FIG. 2, as the number of channels increases and the number of systems 100 increases, the number of dedicated control shelves CS required for a full communication basestation can become quite large. This occurs since the stations 205-208 are provided as dedicated stations for each of the channels received by R.sub.x coupler 204 or transmitted by T.sub.x combiner 202.
Multi-channel basestations are thus old. Also, the general concept of zero-if or receivers with multi-channels is old. The basic concepts of prior configurations have, however, introduced several problems that up to now seemed insurmountable.
One prior configuration proposed that several channels (perhaps 8 or so) be block received and then an FFT be run on the full block. By a technique of frequency shifting in the frequency domain this configuration proposed to extract the specific channel desired.
Another proposal involved placing the multi-channel block into an A/D converter with a large dynamic range and then sorting the data by a DSP.
The problems that occur with these methods are listed below:
DSP's to sample and sort through a +/-10 MHz band of signals are not available with the speed or MIPS to do anything of value. The sample rate for the interrupts would consume virtually all the MIPS available, much less allow any real processing to occur. PA1 The inherent dynamic range of signals to extract is potentially greater than 100 dB with many signals at near thermal noise levels. Known A/Ds are not capable of accurately digitizing this range, much less have the quantization resolution to not add its own noise to lower level signals. Therefore some gain would have to be placed before the A/D. Once gain is added, the overall dynamic range is reduced since very strong signals could then overwhelm the circuits before a very small signal could be extracted.